In certain applications of radiofrequency information transmission, it has been noted that the transmit or receive antenna could have an impedance strongly dependent on conditions outside the antenna, and dependent notably on the medium in which the antenna is placed.
For example, in medical telemetry, it might be required to introduce the antenna into a probe placed in the human body, and the impedance then depends strongly on the biological medium in which the antenna is situated. It depends on the electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscles, fat) or the liquid medium (blood, other liquids) in which the antenna may be immersed.
Even in more conventional applications of radiofrequency transmission (mobile telephony, etc.) the impedance of the antenna may vary.
A transmit (respectively receive) chain comprises at least one amplifier with which one or more filters may be associated.
Generally, the antenna impedance variations are particularly noticeable for antennas of very small dimensions having a high quality coefficient, used in applications with high miniaturization constraints.
These impedance variations may give rise to losses termed mismatch losses: these losses result from the fact that the transmit chain which feeds the antenna, or the receive chain which receives a signal from the antenna, is in general designed to have optimal performance when it is loaded (at output for the transmit chain or at input for the receive chain) by a well determined nominal impedance; its performance is degraded when it is loaded by an impedance differing from its nominal value. The mismatch losses may reach 40 dB.
This is why it has already been attempted to interpose between the output of the transmit chain and the transmit antenna (and it could also be done at the input for a receive antenna) an impedance matching network which causes the transmit chain to see a different impedance from that of the antenna and preferably equal to the nominal value for which it was designed, for example 100 ohms or 500 ohms. The matching network is tunable, that is to say its elements, capacitive and/or inductive, have adjustable values to take account of the antenna environment conditions so that the matching is the best possible whatever the circumstances.
In patent application US 2009-0066440 there is proposed a method of automatic impedance matching in a transmit or receive chain, in which method the amplitude and the phase of the current and of the voltage at the output of the transmit chain (or at the input of the receive chain) are detected at one and the same time. The ratio of the voltage to the current is representative of the load impedance Zm seen by the chain loaded by the assembly of the matching network and impedance antenna Zant. The load impedance Zm is measured and the antenna impedance Zant is calculated on the basis of the measured load impedance Zm and of the impedances of the matching network whose configuration is known at the moment of the measurement, and finally the modification that has to be applied to one or more of the impedances of the matching network, so that the impedance seen by the amplifier becomes matched to the nominal impedance of the amplifier in the existing conditions of the environment of the antenna, is calculated.
In applications where it is enough for the impedance matching to be done once, or in applications where it suffices for the matching to be done each time the transmit or receive chain is switched on, this method operates well.
But in other applications, it may happen that the environment of the antenna changes during use of the chain and requires a new matching without waiting for switch off and switch on again. Such is the case for example in a portable telephone whose environment may change while it is turned on (at least when idle) and even while a telephone conversation is in progress. For these applications, it is desired to effect immediate matching between the impedance of the antenna and the impedance of the transmit circuit or of the receive circuit.
The circuit of the aforementioned patent application does not actually make it possible to do so without troublesome interaction between the transmit/receive chain and the means for calculating matching.